BAGHDAD, Iraq (CNN) -- Fighting between Iraqi security forces and supporters of Shiite cleric Muqtada al-Sadr left 50 dead in the southern city of Basra and spread to several Baghdad districts Tuesday, Iraqi officials said.

Iraqi Sadrist Shiite clerics rally in the holy city of Najaf in central Iraq on Tuesday.

The fighting erupted as al-Sadr's political organization launched a nationwide civil disobedience movement to protest recent arrests of its members.

The discord threatens to unravel a much-praised cease-fire by the cleric's militia, the Mehdi Army, which U.S. commanders have credited with helping ease the sectarian warfare that gripped Iraq in 2006.

An official with Basra's Provincial Council, speaking on condition of anonymity, told CNN that at least 50 people were killed and 120 wounded Tuesday.

The dead included Iraqi troops, police, civilians and militiamen, the official said.

In addition, at least 30 Iraqi security force members were kidnapped by militia fighters in Basra, he said.

Iraqi Prime Minister Nuri al-Maliki is personally overseeing efforts to restore order in Basra, said Ali Hadi, an official in the prime minister's office.

He said al-Maliki met with high-ranking Iraqi security officers ahead of a push against militia fighters, which was launched early Tuesday.

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The provincial government official said the prime minister arrived without advance notice, and provincial officials were unaware of plans for Tuesday's operation. He said explosions could still be heard late Tuesday, but an Interior Ministry official in Baghdad said security forces were in control of most of the city by Tuesday evening.

Residents of Basra, Iraq's second-largest city and its major oil port, held demonstrations earlier this month to demand better security. Kidnappings, murders and thefts have risen since British troops handed over responsibility for the province to Iraqi soldiers and police in December and withdrew to a base at the city's airport.

In addition, violent rivalries among Shiite factions have spread throughout southern Iraq, with Mehdi Army fighters tangling frequently with the Badr Brigade, the militia of the Islamic Supreme Council of Iraq -- which has a strong presence in Iraqi security forces. Al-Sadr's movement, the ISCI and the Fadhila party all wield influence in Basra.

Nassar al-Rubaie, an official in al-Sadr's political movement, linked Tuesday's attacks to provincial elections slated to take place October 1.

"The police and army forces are used for political reasons," al-Rubaie said.

Al-Sadr announced a six-month cease-fire in August, after fighting between his followers and Badr forces in the Shiite holy city of Karbala left more than 50 dead. He extended it for another six months in February, but the Americans and the Iraqi Army have continued to target Mehdi Army followers they accuse of ignoring the agreement.

The Mehdi Army launched two uprisings against U.S. troops in 2004. Leaders of his political movement say the recent raids by U.S. and Iraqi forces have unfairly targeted militia members, and al-Rubaie said government security forces comprise rival militias that have killed, tortured and arrested innocents.

A Basra city council official said that the fighting erupted when security forces entered Mehdi Army strongholds, where militiamen were armed with machine guns, grenades, rockets and mortars. Iraqi security forces picked up 40 suspect militiamen involved in planting bombs Tuesday, the Interior Ministry official said.

Militiamen used roadside bombs to bring down part of a bridge in northern Basra on Tuesday, witnesses and police reported, hindering the movement of security forces.

In Baghdad, meanwhile, Mehdi Army fighters battled Iraqi national police units in eastern and southwestern Baghdad, the Interior Ministry official told CNN. At least six people were wounded, including two police officers, the official said.

Clashes occurred in al-Hamza Square at the edge of Sadr City, the cleric's power base in eastern Baghdad, and spread to more than half a dozen other neighborhoods.

Attackers wielding small arms in the western neighborhood of Tobchi attacked the office of Grand Ayatollah Ali al-Sistani, Iraq's top Shiite cleric, the official said. Iraqi security forces have gained control of the area, and the office is under their protection.

In addition, rockets and mortar shells rained down on U.S. troops around Baghdad, killing one American soldier, a U.S. military spokesman said. A dozen attacks targeted four U.S. bases and the International Zone, the fortified enclave in the heart of Baghdad that houses American and Iraqi government offices, Lt. Col. Steven Stover said.

The Interior Ministry said civil disobedience tactics were seen in five Baghdad neighborhoods.

In the southern Baghdad neighborhood of Abu Disher, news footage showed empty streets, closed stores and empty schools, and a few dozen protesters were seen taking to the streets. Signs reading "Yes yes Iraq" and "No no America" were tacked up on walls, as was a sign saying "no" to government militias, a reference to the Badr Brigade.

Col. Steve Boylan, a U.S. military spokesman in Baghdad, said American commanders don't frown on such a civil disobedience campaign "as long as it's peaceful."

"We have no problem with that," Boylan said. "Shouting is OK. Shooting is not."

Mehdi Army members also appeared on the streets of Mahmoudiya and Yusufiya, towns south of Baghdad, in a show of force. And in Kut, local authorities imposed a curfew in light of the fighting in Baghdad and Basra.

Other developments

• The U.S. soldier who died Tuesday was on patrol in the largely Sunni district of Adhamiyah when a mortar attack struck, Stover said. The fatality brings the number of U.S. dead in the five-year-old war to 4,001, including eight Defense Department civilians.

• Earlier clashes occurred in al-Hamza Square at the edge of Sadr City, the cleric's power base in eastern Baghdad. In Maamel, also in eastern Baghdad, gunmen attacked an Iraqi police checkpoint and kidnapped six police officers, an Interior Ministry official told CNN.

• At least four al Qaeda in Iraq members were killed in volatile Diyala province on Sunday in two military raids, the U.S. military said Tuesday.

• The military also reported a roadside bombing on Sunday that killed two children and wounded two civilians. It occurred in Khatoon, north of the Diyala provincial capital of Baquba.

Diyala is an ethnically and religiously mixed province that stretches north and east of Baghdad and borders Iran.

• The U.S. military also said it had determined that six people killed and two people wounded in an American helicopter strike near Samarra on Saturday were prospective pro-U.S. militia members -- not "terrorists" as previously suspected.E-mail to a friend

CNN's Mohammed Tawfeeq and Jonathan Wald in Baghdad and Joe Sterling in Atlanta contributed to this report.